This study will investigate mechanisms whereby the human pregnancy survives despite its similarity to a reciprocal allograft relationship. The major hypotheses to be explored in order to better understand the immunologic relationship of mother and fetus are: (1) placental syncytiotrophoblast is an antigenically neutral zone that provides a barrier to maternal rejection, (2) cellular immunity of maternal lymphocytes is blocked by humoral factors in the placental bed, and (3) the developing human fetus possesses considerable immunocompetence which affords protection against transplacental passage of foreign antigens. The presence or absence of HL-A antigens on syncytial trophoblast will be investigated with a newly developed indirect immunohistochemical technique. The role of the hormone, hCG, in modulating the in vitro lymphocyte reactivity including: mitogen induced activation, allogeneic interactions, and lymphocyte mediated cytotoxicity will be examined. The immunocompetence of both human and murine fetal hepatic lymphocytes will be analyzed during ontogenesis. Effector cell function of human fetal hepatic lymphocytes in T and B cell mediated cytotoxicity tests will be studied. Finally, precisely timed murine gestants of defined genetic strains will be employed to probe the emergence of cells responsive in allogeneic cell interactions prior to thymic lymphopoesis.